Rename to self-hosted in backend

[chadwhitacre]

Part of getsentry/self-hosted#796, split out from #30528 by request.

Follow up is #30558.

[chadwhitacre]

On this PR, maintain key name used by frontend.

[chadwhitacre]

This is all wrong. I need to actually test this in getsentry. How do I do that?

The result of this PR will be to always set SENTRY_SELF_HOSTED to true, regardless of the value of SENTRY_ONPREMISE in a downstream project. Since SENTRY_SELF_HOSTED then becomes the config passed out to the frontend, if I merge/deploy this then I will effectively turn SaaS into a self-hosted instance.

[dashed]

@chadwhitacre we can add SENTRY_SELF_HOSTED = False here https://github.com/getsentry/getsentry/blob/40972e7b6f8c369da2324a42d01196040d53e2ca/getsentry/conf/settings/defaults.py#L29

[chadwhitacre]

The better way to do this is to:

1. Set SENTRY_SELF_HOSTED in the downstream apps (getsentry and single-tenant) as a no-op duplicate. It sits right next to SENTRY_ONPREMISE but nothing reads it.
2. Modify the upstream (i.e., sentry, here) to use/read SENTRY_SELF_HOSTED on the backend (basically this here PR here).
3. Remove SENTRY_ONPREMISE from the downstreams.
4. Modify the upstream to set isSelfHosted as a no-op duplicate on the frontend.
5. Modify the downstreams to read from isSelfHosted.
6. Modify the upstream to read from isSelfHosted and remove isOnPremise.

The crux is that downstreams set the backend SENTRY_FOO but consume the frontend isFooBar, while upstreams consume the backend and set the frontend. The right approach looks like this:

backend  add    downstream
backend  add    upstream
backend  remove upstream
backend  remove downstream

frontend add    upstream
frontend add    downstream
frontend remove downstream
frontend remove upstream

[chadwhitacre]

I've folded this back into getsentry/self-hosted#796. To be continued over there ...

Dismissing my approval until the new strategy is in place